Tag Archives: testing

Some Progress – AT LAST! – With Figuring the 16.5″ f/4.5 Thin Mirror That Headlines This Blog

10 Saturday Nov 2018

Posted by in astronomy, Hopewell Observatorry, Optics, Safety, Telescope Making

≈ Leave a comment

Tags

, , , , , , , , , ,

I have been wrestling with this mirror for YEARS. It’s not been easy at all. The blank is only about twice the diameter of an 8″ mirror, but the project is easily 10 times as hard as doing an 8-incher. (Yes, it’s the one in the photo heading this blog!)

Recently I’ve been trying to figure it using a polishing/grinding machine fabricated by the late Bob Bolster (who modeled his after the machine that George Ritchey invented for the celebrated 60″ mirror at Mount Wilson over a century ago). That’s been a learning exercise, as I had to learn by trial and error what the machine can and cannot do, and what strokes produce what effects. The texts and videos I have seen on figuring such a large mirror with a machine have not really been very helpful, so it’s mostly been trial and error.

My best results right now seem to come from using an 8″ pitch tool on a metal backing, with a 15 pound lead weight, employing long, somewhat-oval strokes approximately tangential to the 50% zone. The edge of the tool goes about 5 cm over the edge of the blank.

This little movie shows the best ronchigrams I have ever produced with this mirror, after nearly 6 hours of near-continuous work and testing. Take a look:

And compare that to how it used to look back in September:

 

Also compare that to the theoretically perfect computed ronchigrams from Mel Bartels’ website:

perfect theoretical ronchigrams for guy's 42 cm mirror

Part of the reason this mirror has taken so long is that after grinding and polishing by hand some years ago, I finally did a proper check for strain, and discovered that it had some pretty serious strain. I ended up shipping it out to someone in Taos, New Mexico who annealed it – but that changed the figure of the mirror so much that I had to go back to fine grinding (all the way back to 120 or 220 grit, I think), and then re-polishing, all by hand. I tried to do all of that, and figuring of the mirror, at one of the Delmarva Mirror Making Marathons. It was just too much for my back — along with digging drainage ditches at Hopewell Observatory, I ended up in a serious amount of pain and required serious physical therapy (but fortunately, no crutches), so this project went back into storage for a long, long time.

Recently I’ve tried more work by hand and by machine. Unfortunately, when I do work by hand, it seems that almost no matter how carefully I polish, I cause astigmatism (which I am defining as the mirror simply not being a figure of rotation) which I can see at the testing stand as Ronchi lines that are not symmetrical around a horizontal line of reflection. (If a Ronchi grating produces lines that look a bit line the capital letters N, S, o Z, you have astigmatism quite badly. If astigmatism is there, those dreaded curves show up best when your grating is very close to the center of curvature (or center of confusion) of the central zone.

Using this machine means controlling or guessing at a LOT of variables:

  1. length of the first crank;
  2. length (positive or negative) of the second crank;
  3. position of the slide;
  4. diameter of the pitch lap;
  5. composition of the pitch;
  6. shape into which the pitch lap has been carved;
  7. amount of time that the lap was pressed against the lap;
  8. whether that was a hot press or a warm press or a cold press;
  9. amount of weight pushing down on the lap;
  10. type of polishing agent being used;
  11. thickness or dilution of polishing agent;
  12. temperature and humidity of the room;
  13. whether the settings are all kept the same or are allowed to blend into one another (eg by moving the slide);
  14. time spent on any one setup with the previous eleven or more variables;

Here is a sketch of how this works

bolster's ritchey-like machine

Latest Ronchi or Knife-Edge Tester for Mirrors and Other Optics Using a WebCam

07 Friday Sep 2018

Posted by in astronomy, Optics, science, Telescope Making

≈ Leave a comment

Tags

, , , , , , , , ,

Here is the latest incarnation of my webcam Ronchi and knife edge (or Foucault) tester. It’s taken quite a few iterations to get here, including removing all the unnecessary parts of the webcam. I attach a still photo and a short video. The setup does quite a nice job of allowing everybody to see what is happening. The only problem is setting the gain, focus, exposure, brightness, color balance, contrast, and so on in such a way that what you see on the screen resembles in any way what your eye can see quite easily.

IMG_1335

Calculations with a Curious Cassegrain

08 Sunday Oct 2017

Posted by in astronomy, flat, Hopewell Observatorry, optical flat, Optics, Telescope Making

≈ Leave a comment

Tags

, , , , , , , ,

I continue to try to determine the foci of the apparent hyperbolic primary on the Hopewell Ealing 12inch cassegrain, which has serious optical problems.

My two given pieces of information are that the mirror has a radius of curvature (R) of 95 inches by my direct measurement, and its Schwarzschild constant of best fit,(generally indicated by the letter K)  according to FigureXP using my six sets of Couder-mask Foucault readings, is -1.112.

I prefer to use the letter p, which equals K + 1. Thus, p = -0.112. I decided R should be negative, that is, off to the left (I think), though I get the same results, essentially, if R is positive, just flipped left-and-right.

One can obtain the equation of any conic by using the formula

Y^2 – 2Rx + px^2 = 0.

When I plug in my values, I get

Y^2 + 190x -0.112x^2 = 0.

I then used ordinary completing-the-square techniques to find the values of a, b, and c when putting this equation in standard form, that is something like y^2/a^2 – x^2/b^2 = 1

Omitting some of the steps because they are a pain to type, and rounding large values on this paper to the nearest integer (but not in my calculator), I get

I got

y^2 – 0.112(x – 848)^2 = – 80540

and eventually

(x – 848)^2 / 848^2 – y^2 / 248^2 = 1

Which means that a is 848 inches, which is over 70 feet, and b is 284 inches, or almost 24 feet. Since a^2 + b^2 = c^2, then c is about 894. And the focal points are 894 inches from the center of the double-knapped hyperboloid, which is located at (848, 0), so it looks a lot like this:

cass equations

Which of the two naps of this conic section is the location of the actual mirror? I suppose it doesn’t make a big difference.

Making that assumption that means that the foci of this hyperbolic mirror are about 894 – 848 = 46 inches from the center of the primary mirror. I don’t have the exact measurement from the center of the primary to the center of the secondary, but this at least gives me a start. That measurement will need to be made very, very carefully and the location of the secondary checked in three dimensions so that the ronchi lines are as straight as possible.

It certainly does not look like the common focal point for the primary and secondary will be very far behind the front of the secondary!

Bob Bolster gave me an EXTREMELY fast spherical mirror that is about f/0.9 and has diameter 6 inches. I didn’t think at first that would be useful for doing a Hindle sphere test, since I thought that the focal point in back of the secondary would be farther away. But now I think it will probably work after all. (Excellent job as usual, Bob!) (I think)

 

Figuring (parabolizing) Your Mirror

16 Tuesday Dec 2014

Posted by in History, Telescope Making

≈ Leave a comment

Tags

, , ,

5. Figuring and Testing

 A. Polishing pads do a great job of polishing out the pits, but they tend to leave a rough surface that is not a true paraboloid or even a section of a sphere, unless you are very, very lucky. Most folks will switch to a pitch lap for the figuring process, which involves removing sub-microscopic amounts of glass from various zones on your mirror, in order first to make it into a section of a sphere, and then into the bottom of a paraboloid – the only geometric figure that will reflect all of the rays that come from distant stars onto a single focal point. Many treatises have been written about figuring, and I’m not going to add to that list. “Understanding Foucault” by David Harbour gives an excellent explanation of the figuring process, as does Mel Bartels here. However, here are some of the basics:

B. You will need a pitch lap, made either of Gugolz or Acculap or Tempered Burgundy pitch. The first two are synthetic products whose composition is probably secret; the third one is made from the sap of coniferous trees. I’m not going to describe the process of making a pitch lap here, but I combine some of the methods of Carl Zambuto and John Dobson when I make a new one; you can watch it as we do it for you. It’s much less work if you can use a pitch lap that was made by or for someone else who has finished their own project. Sometimes a previously-used pitch lap will have sat around too long and might need to be scraped off and remelted. We generally use roughly square facets, which allow the pitch to flow better and conform itself to your mirror. Without the facets, any high points on the lap have a hard time being lowered. We also tend to use netting or a single-edge razor blade to make minifacets, which further help the lap to conform to the mirror.

C. Pitch is weird stuff. When it’s warm, it flows and it’s very sticky. When it’s cold, it is fairly hard, and you can shatter it with a hammer. If you leave a pencil or a coin on a pitch lap overnight, the next day you can see all of the details of the pencil or coin reproduced perfectly in the pitch. We want the lap to conform itself to your mirror. Then we use the pitch lap to remove all of the irregularities that were left by the polishing pads. So, we warm up the pitch lap to soften it a bit (using a heat lamp or hot water), spread Cerox or rouge onto your mirror, and then press the two together briefly but firmly. We often use some netting to create micro-facets, which help the pitch conform to your mirror even more.

D. The figuring stage can severely try your patience, especially if the tests show a surface that looks weird. But relax! If you persevere and don’t drop the mirror on the ground, success is guaranteed, since it’s just a matter of removing the correct millionth of an inch or two (much less than a micrometer) of glass from the correct zonal ring to achieve near-perfection. One needs to make sure that the lap actually conforms to the mirror; bad contact between the two can cause trouble, and so can a pitch lap that is too hard, too soft, or too thin. All of those are fairly easily fixed, with remarkable results. And we are here to help.

E. One major problem that can affect mirrors is a Turned-Down Edge (TDE). Opinions vary on what causes this dreaded condition, but the evidence suggests to me that TDE appears when the lap is exactly the same size, or slightly smaller, than the mirror itself. To avoid a TDE, do not chip off the parts of the pitch lap that ‘mushroom’ out past the edge. Let them stay there.

F. You will be instructed in a specific set of strokes which will first make your mirror into a sphere. Then, you will be instructed in a different set of strokes that will make your mirror into a good approximation of a perfect paraboloid. Texereau, LeCleire, and many other books describe those strokes. So did Leon Foucault in his 1859 article, which you can find on this blog/website. In our workshop, we will test your mirror frequently with a combination of tests, many of them invented by Foucault but later modified.

G. A very fast qualitative test is the Ronchi test, which you can look up. It gives you almost instant feedback on the presence or absence of bad features like turned-down edge (TDE), zonal defects (high or low rings), astigmatism (lack of symmetry), roughness, and so on. It will tell you whether your mirror is a sphere or not – if the Ronchi lines are perfectly straight, then you have a sphere. If they are not straight, then the test can tell us if your mirror is on the way towards being an ellipsoid with the long axis perpendicular to the mirror (or parallel to it), or a hyperboloid, or your goal, a paraboloid. There are several computer programs that provide simulations of what a perfect mirror should look like under the Ronchi test, but I’ve found you can’t always trust those simulations. RonWin is one such program, and Mel Bartels has another on one of his web pages.

H. A more time-consuming test that I find is necessary is the Foucault test as modified by Andre Couder, also known as the numerical knife-edge test with zones. If the Ronchigram looks good, and the numbers in the knife-edge zonal test are also within acceptable limits, then the mirror is done. A slight modification of this test is known as the Wire Test, which works well on fast mirrors. David Harbour’s article does an excellent job of explaining this test. One can use a pinstick method for marking the zones, or one could write directly on the mirror with a Sharpie, but we use a version that uses cardboard masks with holes cut out at carefully-measured zones.

I. We have tried a number of other tests, such as the double-pass autocollimation test, the Mobsby Null test, and the Bath Interferometer test, and have had difficulties getting good results with them. Therefore, we are continuing to use the Ronchi and Knife-Edge Zonal tests.

J. However, the best test of any mirror is the Star Test, which is the subject of an entire book by Richard Suiter. Some do this in the daylight, using sunlight reflected from very distant insulators on electrical poles. Most do it at night, but it requires steady air (‘good seeing’) and a clear sky as well. The Star Test is much easier to perform if the mirror is already aluminized and in a working telescope, which brings us to….

(Link forward to next section==>)

(<==Link back to previous section)